Rasch Measurement Theory (RMT) Analyses of the Huntington's Disease Everyday Functioning (Hi-DEF) to Evaluate Item Fit and Performance

Abstract

Background: The Huntington's Disease (HD) Everyday Functioning (Hi-DEF) is a new patient-reported outcome (PRO) instrument designed to measure the impact of cognitive impairment on daily functioning in the early stages of HD.

Objective: To assess the measurement properties and finalize item content of the Hi-DEF.

Methods: A cross-sectional, observational psychometric validation study was conducted among individuals with early stages of HD at 9 US centers of excellence. Rasch Measurement Theory (RMT) analysis of the initial draft version of the Hi-DEF (47 items) and subscales (i.e., 'Home', 'At work', 'Driving', and 'Communication') was conducted to examine measurement properties including sample-to-scale targeting, suitability of response scale (ordering of response thresholds), scale cohesiveness (item fit), local independence, and person fit.

Results: 151 participants (mean age 47 years (SD 12), 59% female) were included. Seven items were removed based on dependency and item fit. The remaining 40-item version of the Hi-DEF demonstrated good measurement properties. Across the four subscales, targeting ranged from 49-70% (72% full scale), reliability ascertained by person separation index ranged from 0.53-0.87 (0.92 full scale), response scales were ordered for 25-100% of items (75% full scale), 0-12% items displayed misfit (2% full scale), and 0-1% (2% full scale) item pairs displayed dependency.

Conclusions: Our study supports the psychometric integrity of the Hi-DEF as a reliable and valid new PRO instrument designed to assess the impact of cognitive impairment on daily functioning in the early stages of HD. Future work will evaluate the external validity and utility in clinical trial applications.

Keywords: Hi-DEF; Huntington’s disease; activities of daily living; executive function; patient-reported outcomes; psychometrics.

Fig. 1

Hi-DEF scale (total score) targeting plot. The upper histogram represents the sample distribution for the Hi-DEF scale total score whereas the lower histogram represents the scale item threshold distribution plotted on the same linear measurement continuum. This allows a comparison between the range of cognitive functioning difficulties reported in the sample (upper histogram) and the range of cognitive functioning difficulties measured by the items of the Hi-DEF (lower histogram). Overlap between the ranges of the sample and item threshold distributions indicates the instrument is well-matched and able to measure the construct (cognitive functioning difficulties) within the sample accurately. The curve above the upper histogram represents an inverse function of the standard error associated with each person measurement (the peak of the curve indicating the best point of measurement).

Fig. 2

Targeting plots of Hi-DEF subscales: 1) Home; 2) At work; 3) Driving; 4) Communicating. The upper histogram represents the sample distribution for each Hi-DEF subscale (Home, Work, Driving, and Communicating) whereas the lower histograms represent the item threshold distribution for each subscale plotted on the same linear measurement continuum. This allows a comparison between the range of cognitive functioning difficulties reported in the sample (upper histogram) and the range of cognitive functioning difficulties measured by the items of the Hi-DEF (lower histogram) for each subscale. Overlap between the ranges of the sample and item threshold distributions indicates the instrument is well-matched and able to measure the construct (cognitive functioning difficulties) within the sample accurately. The curve over the upper histogram represents an inverse function of the standard error associated with each person measurement (the peak of the curve indicating the best point of measurement). These figures illustrate the targeting for each of the instrument subscales. The Hi-DEF subscales capture everyday functioning in different environments and may not be applicable to everyone.

Fig. 3

Hi-DEF full scale targeting plot by TFC score. The upper histogram (variable blocks: colors and patterns indicate TFC score, ranging from TFC 13 to TFC 8, as shown in the key) represents the sample distribution for the Hi-DEF scale, whereas the lower histogram represents the scale item threshold distribution plotted on the same linear measurement continuum.